In recent years, there has been a tremendous increase in the use of wireless communication devices. The increased use has filled or nearly filled existing frequency bands. As a result, new wireless frequency band standards are emerging throughout the world. For example, the existing 1st (1G) and 2nd (2G) generation cellular mobile communication systems operate at:                the AMPS (824-894 MHz) and PCS (1850-1990 MHz) bands in North America;        the GSM (880-960 MHz) and DCS (1710-1880 MHz) bands in Europe; and        the PDC (810-915 MHz) and PHS (1895-1918 MHz) bands in Japan.For future wireless communication systems, such as the emerging 3rd generation (3G) systems or beyond, new spectrum may be allocated around 2 GHz (e.g., already identified 1920-2170 MHz band for UMTS or IMT2000).        
Like cellular mobile communications systems, Wireless Local Area Networks (WLANs) also use various frequency bands. IEEE 802.11b, Bluetooth, and HomeRF operate in the 2.4 GHz ISM band (2.400-2.485 GHz). IEEE802.11a and HiperLAN (in Europe) will use the 5 GHz ISM band (5.15-5.35 GHz and 5.725-5.825 GHz for IEEE802.11a, 5.15-5.25 GHz for HiperLAN1 and 5.15-5.35 GHz for HiperLAN2). Japan has started the development of standards for WLAN devices in the 5 GHz band.
As the frequency standards throughout the world change and evolve, wireless devices that can operate at the old and the new frequency standards are needed.
Increased functionality is another factor that drives the need for wireless devices that can operate at multiple frequencies. New wireless devices may provide multiple functions, but one or more of the functionalities may only be available at a respective one or more different frequencies from the base operating frequency. Thus, there is a need for wireless devices that can operate and implement functionalities at more than one frequency.
Yet another factor that drives the need for wireless devices that can operate at multiple frequencies is the desire of users for multi-functional services that operate at high data speeds including voice, video, and data transmissions. A wireless device may provide such services with automatic access and seamless roaming if the device can operate across multiple frequency bands.
The antenna is a key component in the realization of such a multi-mode wireless device. It is desirable for an antenna used in a multi-mode wireless device to include broadband performance for use in successive bands. It is also desirable for such an antenna to have multi-band performance for separated bands including far-separated bands. In addition to broadband and multi-band performance, it is desirable for such an antenna to be of a small size, a simple structure, and be of lightweight materials so as to be easily mounted in a handheld terminal with relatively low cost. Further, the radiation patterns in all service bands of such an antenna should be omni-directional and polarization-mixed to adapt to land-mobile propagation environments.
In recent years, a great number of new antenna structures have been developed for dual-band or triple-band operations in wireless communication handsets. A simple way to realize dual-band operation is to directly feed two antenna elements, each of which has a separate resonant frequency. For example, a combination of a monopole and a helical antenna, where the monopole is placed through the middle of the helix in the axial position and is simply connected to the end of the helix, has been successfully applied in GSM/DCS bands. Directly feeding two monopoles with different lengths can also result in two resonant frequencies. Another dual-band operation includes electromagnetically coupling two separate radiating elements. A coupling dual-band dipole antenna has been developed for WLAN applications in the 2.4 and 5.2 GHz bands. By coupling a rectangular element at the high frequency and an L-shaped element at the lower frequency, a dual-band operation was achieved for a planar inverted-F antenna (PIFA). The triple-band operation of the PIFA was implemented by adding one more L-shaped radiator.
Usually, a dual-band or triple-band antenna has a narrow bandwidth at each band. In order to achieve a broadband multi-band operation, some specific techniques or additional structures have to be incorporated. For instance, a broadband dual-band operation could be realized by properly notching a rectangular patch. The bandwidth of the higher band for a dual-band PIFA was increased by adding one more resonator. By introducing a stacked element, by making the longer and shorter dipoles resonate, respectively, at slightly below and slightly above the center frequency, or by adding some parasitic structures, the bandwidth at one of the two bands of a dual-band antenna may be increased. Yet, broadband performance is desired at every band of a multi-band antenna.
Accordingly, there is a need for multi-band broadband antennas. In particular, there is a need for multi-band broadband antennas that are of small size, simple structure, and lightweight materials so as to be easily mounted in a handheld terminal with relatively low cost.